1. Field of the Invention
The present invention relates to a camera in which an incident optical beam is divided for use in both photographing and observation of an image.
2. Description of the Prior Art
As a conventional example, an article in the June, 1994 issue of the "Photographic Industries" magazine, pages 12 to 15, describes a camera which is capable of changing the path of an incident light beam for the purpose of photographing and observing an image at the same time. According to the article, the camera uses one of the divided light beam for taking a photograph on a silver salt or a silver halide film, and the other for picking up an image on a CCD (Charge Coupled Device). The CCD reportedly is of a 1/2-inch type.
An attempt to simultaneously take a photograph on a silver salt film and pick up an image on an image pickup device is subject to a problem as described below. Generally, an image pickup device having a considerably small image area is used for this purpose, since an image pickup device having a wide image area is expensive. As seen from a comparison shown in FIG. 15, even the image area of a comparatively large image pickup device of a 1-inch type, for example, is substantially smaller than a image area on a film. Accordingly, in order to adjust the spread of an incident light beam to the image area, it is necessary to use a reduction optical system such as a relay optical system. Considering an image pickup device of a 1/4- to 1/2-inch type is generally used, the relay optical system needs to have a magnification of 1/10 to 1/4. However, it is often impossible to use a relay optical system capable of supplying sufficient light, since there is only limited space for extra components inside a camera body.
As a consequence of the above limitations, the following problem arises. As shown in FIG. 14, if a relay optical system of a magnification .beta.=1/4 and of an open aperture value F=1.4 (represented also by a) is used, a light beam incident from a taking lens TL enters the relay optical system RL without diffusing on a primary image plane IMG1. Accordingly, of the light beam incident from the taking lens TL, only a portion corresponding to a/.beta.=5.6, that is, F=5.6 is used as a light beam which passes through the relay optical system RL to form a secondary image IMG2, while the remaining light beam (indicated with hatching in the drawing) passing outside the light beam corresponding to F=5.6 does not enter the image pickup device. As a result, if the aperture is adjusted to be wider than F=5.6 by use of a taking lens of an open aperture value less than F=5.6, the amount of light incident on a silver salt film does change accordingly, while the amount of light incident on the image pickup device remains the same. This leads to that, although the exposure of a silver salt film can be adjusted by adjustment of aperture, the exposure of a image pickup device cannot be adjusted in the same manner; a different adjustment needs to be devised.
Moreover, if the aperture of the taking lens is adjusted to be narrower than F=5.6 when simultaneous taking of a photograph on a silver salt film is attempted during exclusively performed picking up of an image on an image pickup device, the amount of light incident on the image pickup device is also affected, disturbing proper picking up of an image.
As another conventional example, Japanese Laid-Open Patent No. S63-284527 discloses a camera which takes a photograph on a silver salt film and picks up an image on an image pickup device, and in which an image thus taken by the image pickup device can be reproduced and observed through a viewfinder. In a camera of this type, it is possible to preview an image frame taken on a silver salt film by displaying a stationary image in an electronic viewfinder.
As still another conventional example, a camera is well known which stops down the aperture of a taking lens without taking a photograph on a silver salt film so that picture effects (such as a depth-of-field effect) achieved by stopping down the aperture can be previewed through an optical viewfinder, that is, a single-lens reflex camera with a so-called preview mechanism.
Although a preview mechanism adopted conventionally in a single-lens reflex camera allows previewing of picture effects, an image provided by such a preview mechanism is dark, because the image is observed with the aperture stopped down. Therefore, the conventional preview mechanism is defective because it does not offer a clear image when the aperture is extremely stopped down. Moreover, if a camera is provided with a mechanism for detecting focus by use of a light beam incident through a taking lens, it is impossible to detect focus during previewing, because a light beam to be used for focus detection is obstructed when the aperture is extremely stopped down.
Further, since a conventional preview mechanism does not allow observation of a stationary image, it is difficult to preview an image photographed on a silver salt film, and it is impossible to observe picture effects (such as a shaken-image effect) achieved by a change in shutter speed.
On the other hand, although the camera according to Japanese Laid-Open Patent No. S63-284527 allows previewing of an image frame, it does not allow previewing of picture effects achieved by changes in aperture or shutter speed.